1. Field of the Invention
The present invention relates to a process for removing trace levels of metallic impurities from a photoresist component (e.g., novolak resin) solution. In particular, the present invention is directed to a process of removing trace levels of metallic impurities from such photoresist component solutions by washing the photoresist component solution with a mixture of cyclohexane and isopropyl acetate and acidic aqueous solution; and then washing the photoresist component with mixtures of water and the original solvent.
2. Brief Description of the Prior Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. Generally, in these processes, a thin coating or film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates of printed wiring boards. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam, and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed area of the coated surface of the substrate.
There are two types of photoresist compositions-negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g., a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative-working resist with a developer solution causes removal of the nonexposed areas of the resist coating and the creation of a negative image in the photoresist coating, and thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become more soluble to the developer solution (e.g., a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer solution causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. Again, the desired portion of the underlying substrate surface is uncovered.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. This etchant solution of plasma gases etch the portion of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected and thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining resist layer after the development step and before the etching step to increase it adhesion to the underlying substrate and its resistance to etching solutions.
Positive-working photoresists are generally prepared by blending a suitable alkali-soluble binder resin with a photoactive compound (PAC) which converts from being insoluble to soluble in an alkaline aqueous developer solution after exposure to a light of energy source. The most common class of PAC's employed today for positive-working resists are napthoquinonediazide (DNQ) esters of a polyhydroxy compound.
Positive-working photoresist compositions are currently favored over negative-working resists because the former generally have better resolution capabilities and pattern transfer characteristics.
The quality of photoresists can be improved by substantially reducing the amount of contaminating metal ions in the photoresists. These metallic impurities include ions of iron, sodium, barium, calcium, magnesium, copper, and manganese as well as other metals. In positive-working resists, these impurities are mainly attributable to the binder resin component in the photoresist. The binder resin in positive-working resists is generally a phenolic formaldehyde novolak resin. Typical novolak resins used today for positive-working resins are made from various mixtures of cresols, xylenols, and trimethylphenols which are condensed with an aldehyde source (e.g., formaldehyde). The contaminating metal ions get into these resins primarily as a result of their preparation process. Moreover, the free phenolic OH groups in novolak resins promote the incorporation of metal ions therein by proton exchange and by complexing on the polar groups. In other words, once metallic ion impurities get into a novolak resin, it is difficult to remove them.
Water washing of impure novolak resin dissolved in an organic solvent results in only a minor purifying effect. Similarly, techniques involving volatization of the metal ions are impracticable.
The prior art is filled with numerous techniques for reducing the amount of metal impurities in photoresist components. Some of these teachings including the following:
1. U.S. Pat. No. 2,865,875, which issued to Hyman et al., on Dec. 23, 1958, is discussed with producing "low ash" phenol-formaldehyde resins and their preparation. Note that the patent teaches that phenol and formaldehyde may be reacted with two different types of alkaline catalysts (i.e., a fixed alkali metal catalyst such as caustic soda or a volatile nitrogen-containing catalyst such as ammonia). The patent states that a drawback to the use of the fixed alkali catalyst is the objectionable presence of the fixed alkali in the final resin. The patent described a prior art method for producing "filtered resins" which involved reacting phenol and formaldehyde with a mixed alkali and then after condensation has occurred, adding a precipitating acid such as phosphoric or oxalic acid. This causes a substantial part of the free alkali in the reaction mixture to be precipitated. That precipitate can then be filtered out. The Hyman et al. patent alleges that this prior art method always leaves behind residual traces of the salt formed which is high enough to adversely affect the quality of the product.
Hyman et al.'s invention instead involved (1) carrying out the phenol-formaldehyde condensation in a conventional manner with a fixed alkali catalyst; (2) then removing that fixed alkali catalyst in whole or in part by use of an cationic ion exchanger; and (3) and if pH of the resin solution goes below 4, adjusting the pH of the resin solution upward to pH 4-8 by addition of further alkali. Suitable cationic ion exchangers mentioned in the patent included Nalcite, Dowex, Amberlite, and Zeo Karb.
2. U.S. Pat. No. 3,067,172, which issued to Carlstrom on Dec. 4, 1972, teaches removing metal from a phenol formaldehyde resole resin by passing a solution of that resin through a column of a cation exchange material which is insoluble in that solution and saturated with ammonium ions in exchanging position.
3. U.S. Pat. No. 3,432,453, which issued to Gladney et al. on Mar. 11, 1969, teaches a process for removing magnesium, barium or strontium ions from an aqueous solution of phenol-formaldehyde resin comprising (1) adding a soluble ammonium salt (e.g., sulfate, phosphate or carbonate) to said aqueous solution in an amount to bring the pH of said solution to about 5.0 to 6.5, the anion of said ammonium salt capable of forming an insoluble salt with said Mg, Ba or Sr ions; and (2) then raising the pH to 7 of said aqueous solution by addition thereto of ammonia.
4. U.S. Pat. No. 4,033,909, which issued to Papa on Jul. 5, 1977 teaches the removal of ionic species from phenolic resoles by treatment thereof with the free acid form of a cation exchange resin and the hydroxyl form of a strongly basic anion exchange resin.
5. U.S. Pat. No. 4,725,523, which issued to Miura et al. on Feb. 16, 1988, teaches the addition of oxalic acid dihydrate to a novolak solution (see Synthesis Example No. 5, in column 4).
6. U.S. Pat. No. 5,073,622, which issued to Wojtech et al. on Dec. 17, 1991 and is assigned to Hoechst AG, claims process for the preparation of novolak resins having a reduced amount of metal ions, comprising the steps of: (1) dissolving a conventional novolak resin in an organic solvent or solvent mixture in a concentration of about 25 to 50% by weight and then (2) contacting the resultant solution at least once with an acidic compound (e.g., formic acid, acetic acid, oxalic acid, malonic acid, glycolic acid, lactic acid, tartaric acid, or citric acid).
7. U.S. Pat. No. 5,075,193, which issued to Dreselz et al. on Dec. 25, 1991, claims the use of microcrystalline cellulose to remove metal impurities from a solution of a naphthaquinonediazide DNQ compound by absorbing the DNQ compound into the microcrystalline cellulose.
8. U.S. Pat. No. 5,080,997, which issued to Hioki et al. on Jan. 14, 1992, claims a process for preparing a positive resist composition, which process comprise the steps of: reacting a quinone diazide sulfonyl halogenide with a phenol compound in a condensation reaction solvent to form a condensation reaction mixture; mixing the condensation reaction mixture with a solution of an alkali-soluble resin in a resist solvent without isolating a quinone diazide sulfonyl ester from the condensation reaction mixture to form a second mixture; evaporating said condensation reaction solvent from the second mixture to form a third mixture; washing the third mixture with water to form a fourth mixture; and evaporating the water from said fourth mixture to prepare said positive resist composition.
9. U.S. Pat. No. 5,116,715, which issued to Roland et al. on May 26, 1992, teaches the treatment of an aqueous solution of the sodium salt of 1,2-naphthaquinonediazide-5-sulfonic acid (5-DNQ) with a cationic ion exchange resin to make a free-acid 5-DNQ compound for use in making a capped novolak. See col. 8, lines 58-64.
10. U.S. Pat. No. 5,378,802, which issued to Honda on Jan. 3, 1995, teaches a method of removing ionic impurities from a resist component, comprising the steps of: (a) dissolving said resist component in a solvent; (b) contacting said resist component solution with a fibrous ion exchange resin for a sufficient amount of time to remove at least a portion of said ionic impurities onto said fibrous ion exchange resin; and (c) separating said fibrous ion exchange resin bearing said ionic impurities from said resist component solution.
11. European Patent Application 0251187-A2, which was published on Jan. 7, 1988, claims a method for purifying novolak resins by (1) dissolving the novolak resin in a solvent having certain water-insolubility characteristics; (2) extracting the resulting solution with an acidic aqueous solution to reduce the metal content of the resin and then subjecting the resulting extracted solution to centrifugal separation.
12. Czechoslovakian Patent No. CS 259,458, which was published on Apr. 14, 1989, teaches that positive photoresist containing an o-naphthoquinone diazide sensitizer and a novolak resin in an organic solvent may be freed of salts and ions by treatment with anion and cation exchangers at 5.degree.-35.degree. C.
When ethyl lactate, ethyl-3-ethoxy propionate, methyl-3-methoxypropionate, or propylene glycol methyl ether acetate or mixtures thereof are used as a resist component solvent, it is difficult to wash the resultant resist component solution with an aqueous acidic solution (e.g., aqueous oxalic acid solution) to extract trace metal impurities because these organic solvents are very water soluble. When they are used in such washing operations, a portion of them (with a portion of the resist component dissolved there) enter the aqueous phase of the two-phase mixture. Thus, separation of the organic phase and the aqueous phase in such cases results in a loss of the resist component and resist component solvent with the later discarded aqueous phase.
Today, the reduction of metallic impurities is critical to the production of advanced photoresist compositions, particularly those using i-line imaging for the production of 32 Mbit and 64 Mbit and larger Mbit semiconductor devices. Specifically, there is now a need to produce photoresist compositions which contain less than 10 parts per billion of each metallic ion impurity. The process of the present invention is an answer to that need.